The usual railroad freight cars in use in the United States have a body which is supported at each end by a conventional four wheel two axle railroad bogie or truck. While such cars are capable of carrying most cargo transported throughout the country there are some loads which are much too heavy to be transported by such cars, even with trucks of 100 ton capacity. Some such cargoes are electric generators, nuclear reactors, heavy walled pressure vessels and special equipment pertaining to national defense and space exploration.
To transport extra heavy loads of the described types special railroad cars have been developed which utilize more than two trucks per car so as to distribute the load between the trucks and over a greater area of track roadbed. One way of distributing the load is to connect a pair of railroad trucks by means of a span bolster, each end of which has a body bolster supported by a truck bolster. The top center of the span bolster has a center bowl which receives a center plate mounted on the bottom of the car body, which may be of various kinds including a flat car body. Span bolsters of the described type are shown in Hackbarth et al U.S. Pat. No. 4,160,420, FIG. 1, and Kramlinger U.S. Pat. No. 4,263,852, FIG. 1.
The only commercially available cast span bolster is made by Buckeye Steel Castings from five separate cast steel pieces which are girth seam welded together. The two end body bolsters are separate cast pieces as is the center bolster. These three pieces are joined together by a center sill casting between each end body bolster and the center bolster. Such a span bolster is costly. Also, the center plates and the center bowl are integral with the castings so they cannot be replaced when worn nor can they be repositioned for alignment adjustment after the cast pieces are welded together.
Another type of span bolster has been installed on extra heavy load carrying railroad cars by The Maxson Corporation, St. Paul, Minn. The Maxson span bolster was fabricated from steel plate weldments. However, it was not a successful design in that it occasionally failed in use.